These products cause system degradation because the sludge or varnishlike materials collect at close-fitting, moving parts, such as the spool and sleeve on servo valves. Collection of oxidation products at these points causes sluggish valve response. Metallic Solid Contamination Metallic solid contaminants are usually found in hydraulics systems. The size of the contaminants will range from microscopic particles to those you can see with the naked eye. These particles are the result of the wearing and scoring of bare metal parts and plating materials, such as silver and chromium. Wear products and other foreign metal particles, such as steel, aluminum, and copper, act as metallic catalysts in the formation of oxidation products. Fine metallic particles enter hydraulic fluid from within the system. Although most of the metals used for parts fabrication and plating are found in hydraulic fluid, the major metallic materials found are ferrous, aluminum, and chromium particles. Hydraulic pumps usually contribute the most contamination to the system because of their high-speed, internal movement. Other hydraulic systems produce hydraulic fluid contamination due to body wear and chipping. Hydraulic actuators and valves are affected by contamination. Large metallic or hard nonmetallic particles collect at the seal areas. These particles may groove the inside wall of the actuator body due to a scraping action. Smaller particles act as abrasives between the seals and the actuator body, causing wear and scoring. Eventually, the fluid leaks and the seals fail because the seal extrudes into the enlarged gap between the piston head and the bore of the actuator body. Once wear begins, it increases at a faster rate because wear particles add to the abrasive material. In a similar manner, metallic or nonmetallic parts may lodge in the poppets and poppet-seat portions of valves and cause system malfunction by holding valves open. Inorganic Solid Contamination The inorganic solid contaminant group includes dust, paint particles, dirt, and silicates. These and other materials are often drawn into hydraulic systems from external sources. The wet piston shaft of a hydraulic actuator may draw some of these foreign materials into the cylinder past the wiper and dynamic seals.    The contaminant materials are then dispersed in the hydraulic fluid. Also, contaminants may enter the hydraulic fluid during maintenance when tubing, hoses, fittings, and components are disconnected or replaced. To avoid these problems, all exposed fluid ports should be sealed with approved protective closures. Glass particles from glass bead peening and blasting are another contaminant. Glass particles are particularly undesirable because glass abrades synthetic rubber seals and the very fine surfaces of critical moving parts. FLUID CONTAMINATION Hydraulic fluid can be contaminated by air, water, solvents, and foreign fluids. These contaminants and their effects are discussed in the following text. Air Contamination Hydraulic fluids are adversely affected by dissolved, entrained, or free air. Air may be introduced through improper maintenance or as a result of system design. Air is sometimes introduced when changing filters. You can minimize this kind of contamination by putting hydraulic fluid into the filter holder before reassembling the filter. By doing this, you have introduced less air into the hydraulic system. The presence of air in a hydraulic system causes spongy response during system operation. Air causes cavitation and erodes hydraulic components. Air also contributes to the corrosion of hydraulic components. Water Contamination Water is a serious contaminant of hydraulic systems. Corrective maintenance actions must be taken to remove ail free or emulsified water from hydraulic systems. Hydraulic fluids and hydraulic system components are adversely affected by dissolved, emulsified, or free water. Water may be induced through the failure of a component, seal, line or fitting, poor or improper maintenance practices, and servicing. Water may also be condensed from air entering vented systems. The presence of water in hydraulic systems can result in the formation of undesired oxidation products, and corrosion of metallic surfaces will occur. These oxidation products will also cause hydraulic seals to deteriorate and fail, resulting in leaks. If the water in the system results in the 4-6